Exhaust gas heat exchanger and sealing device for the same

ABSTRACT

An exhaust gas heat exchanger including connection points for the exhaust gas flow, for connecting the exhaust gas heat exchanger to an exhaust gas supply line for supplying a hot exhaust gas and an exhaust gas withdrawal line for withdrawing the exhaust gas flow cooled in the exhaust gas heat exchanger. The exhaust gas flow flows through the exhaust gas heat exchanger in a bundle of exhaust gas guiding pipes in a flow direction. The exhaust gas heat exchanger is provided with at least one coolant supply connection and at least one coolant withdrawal connection. Coolant is guided in a coolant channel in the exhaust gas heat exchanger, inside which it flows around the bundle of exhaust gas guiding pipes. The coolant channel includes at least two regions which differ in terms of the flow direction of the exhaust gas flow by divergent flow directions of the coolant.

The present invention relates to exhaust gas heat exchangers, as areknown in general, and to a sealing device which is suitable particularlyfor use in the case of exhaust gas heat exchangers.

Exhaust gas heat exchangers are used in exhaust gas recirculationsystems in particular for increasing the mass of combusted air taken induring an intake stroke. For this purpose, the density of therecirculating exhaust gas flow has to be increased, which takes place bymeans of cooling of the recirculated gas flow. This usually takes placeby the exhaust gas flow flowing through an exhaust gas heat exchangerwhere it outputs heat to a coolant, such as a cooling liquid.

Exhaust gas heat exchangers are therefore known, through which both anexhaust gas flow and a coolant flow flow. The exhaust gas heat exchangerhas for the exhaust gas flow connection points for connecting the heatexchanger, with, firstly, an exhaust gas supply line being provided forsupplying the hot exhaust gas flow, and an exhaust gas withdrawal linebeing provided for withdrawing the exhaust gas flow cooled in theexhaust gas heat exchanger. In this case, the exhaust gas flow flows ina throughflow direction through the exhaust gas heat exchanger in abundle of exhaust gas guiding tubes. The bundle of exhaust gas guidingtubes serves essentially to enlarge the exchange surface between theexhaust gas flow and the coolant flow. At least one coolant supplyconnection and at least one coolant withdrawal connection are providedfor the throughflow of the coolant flow through the exhaust gas heatexchanger. The coolant is guided here in a coolant channel within whichit flows around the bundle of exhaust gas guiding tubes.

In the case of exhaust gas heat exchangers of this type, the coolantflow is either guided through the exhaust gas heat exchanger in parallelwith and in the direction of the exhaust gas flow or else is guidedthrough the exhaust gas heat exchanger counter to the throughflowdirection of the exhaust gas flow.

The selection of the throughflow direction of the coolant flowing withrespect to the exhaust gas flow is dependent here firstly on structuralconditions, for example the option of permitting the linear compensationfor the thermal expansion of the parts conducting exhaust gas throughthe exhaust gas heat exchanger and secondly in respect of optimizing theheat output of the exhaust gas flow in the heat exchanger. In this case,the optimization is undertaken to the effect that the exhaust gas flowis to output as large a quantity of heat as possible to the coolant.

Furthermore, vibration problems frequently also occur in the case ofexhaust gas heat exchangers. The excitations to vibration that areinitiated in the driving mode of a vehicle and during operation of theinternal combustion engine are also transmitted to the exhaust gas heatexchanger and, for example via the exhaust gas flow, excitations tovibration are also transmitted directly to the parts conducting theexhaust gas flow. In this case, in particular the exhaust gas guidingtubes have a tendency to vibrate, since they generally span a freelength which is as long as possible and can vibrate in this region. Itis therefore the object of the invention to optimize the design of anexhaust gas heat exchanger to the effect that the disadvantages can becompensated for and, at the same time, the quantity of heat output canbe optimized. This object is achieved according to the invention by anexhaust gas heat exchanger according to claim 1.

A further technical problem which occurs in conjunction with exhaust gasheat exchangers is the fluidtight guiding of the exhaust-gas-conductingconnection points out of the exhaust gas heat exchanger. Due to thechanging thermal load, this guiding of them out has to be able tocompensate for an expansion play of the exhaust-gas-conducting parts. Onthe other hand, it is necessary for the tightness to continue to beensured in this region. A particularly favorable design of a sealingdevice of this type can be gathered from the further independent claim.In this case, a sealing device of this type can also be used in the caseof other component leadthroughs, which are to be designed in afluidtight manner, out of another component which is thermally moreheavily or less heavily loaded.

An exhaust gas heat exchanger designed according to the invention has anexhaust gas flow and a coolant flow flowing through it. The exhaust gasheat exchanger has for the exhaust gas flow connection points forconnecting the exhaust gas heat exchanger both to an exhaust gas supplyline for supplying a hot exhaust gas flow and an exhaust gas withdrawalline for withdrawing the exhaust gas flow cooled in the exhaust gas heatexchanger. The exhaust gas flow here flows in a throughflow directionthrough the exhaust gas heat exchanger in a bundle of exhaust gasguiding tubes. Furthermore, a coolant flows in a coolant flow throughthe exhaust gas heat exchanger. For this purpose, at least one coolantsupply connection and at least one coolant withdrawal connection areprovided. In the exhaust gas heat exchanger, the coolant is guided in acoolant channel within which it flows around the bundle of exhaust gasguiding tubes. According to the invention, the coolant channel has atleast two regions which differ in terms of the throughflow direction ofthe exhaust gas flow by diverging throughflow directions of the coolant.

By means of the division of the coolant channel into two regions throughwhich the flow flows in different directions, the transmission of heatfrom the exhaust gas flow to the coolant flow can be improved by thefact that the difference in temperature between exhaust gas flow andcoolant flow is increased by a section twice being available at whichcoolant having the starting temperature is supplied and this not beingin contact with the exhaust gas flow over the entire length of theexhaust gas heat exchanger.

In this connection, according to an advantageous refinement of theinvention, each of the regions is assigned at least one coolant channelsegment, with a coolant channel segment being fluidically connected ineach case to the coolant supply connection and to the coolant withdrawalconnection. The division into coolant channel segments and itsrespective fluidic connection with coolant supply connection and coolantwithdrawal connection permits a simple division of the coolant channelinto subsections having a different throughflow direction.

According to an advantageous refinement, at least one region of thecoolant channel has the flow flowing through it in the throughflowdirection of the exhaust gas flow. A throughflow in the same directionof exhaust gas flow and coolant flow permits a relatively long contacttime between the coolant flowing through and the exhaust gas flowingthrough. By this means, in particular if the difference in temperaturebetween exhaust gas and coolant is very large, a very favorable transferof energy from the exhaust gas flow to the coolant flow is madepossible. According to a further advantageous refinement, there is atleast one region in which the coolant flow flows through counter to thethroughflow direction of the exhaust gas flow. By means of this measure,a difference in temperature which is as constant as possible betweenexhaust gas flow and coolant flow is maintained over the entire lengthof the region. This measure also serves to optimize the transfer ofenergy from the exhaust gas flow to the coolant flow. A direction offlow of the coolant flow transverse to the exhaust gas flow is alsopossible.

According to a preferred refinement of the invention, a region of thecoolant channel which has the flow flowing through it counter to thethroughflow direction of the exhaust gas flow is formed on theapproach-flow side of the exhaust gas guiding tubes and preferably aregion of the coolant channel which has the flow flowing through it inthe throughflow direction of the exhaust gas flow is formed on thedischarge-flow side of the exhaust gas guiding tubes.

According to an alternative refinement, it is provided that a region ofthe coolant channel which has the flow flowing through it counter to thethroughflow direction of the exhaust gas flow is formed on theapproach-flow side of the exhaust gas guiding tubes and preferably aregion of the coolant channel which has the flow flowing through it inthe throughflow direction of the exhaust gas flow is formed on thedischarge-flow side of the exhaust gas guiding tubes.

A further refinement of the invention makes provision for the at leasttwo regions to have a fixed length ratio. According to a firstadvantageous refinement, the length ratio is determined as a function ofthe thermal coefficient of expansion of the material used for producingthe exhaust gas guiding tubes. The length ratio is preferably selectedin such a manner that a fixed relationship is observed in the thermallinear expansion of the two regions, the fixed relationship residing inparticular in a length ratio, which is constant irrespective oftemperature, of preferably one. The effect achieved by this measure isthat the two different regions through which the flow flows have aconstant length ratio irrespective of the temperature of each other.This measure leads in particular to the thermal expansion behavior ofthe exhaust gas heat exchanger being favorably influenced.

Another advantageous refinement makes provision for the length ratio tobe determined in such a manner that in each of the two regions apredetermined portion of the heat energy output overall by the exhaustgas flow is output. Provision may be made here in particular for a smallportion of the quantity of heat output overall to be output, forexample, to the region through which the exhaust gas flow first of allflows, with more than 70%, in particular between 80 and 95%, of thequantity of heat output overall being output, for example, in one of thetwo regions. According to such a refinement of the invention, an exhaustgas heat exchanger is therefore provided by a small quantity of heatbeing output in a first region, which is preferably kept short, whilethe main part of the quantity of heat is exchanged in a subsequent, longsection. This measure makes it possible to distribute the exchange ofenergy to two regions, with each of the regions being optimized in itsfunction. In particular, by means of a design of this type, an overallrelatively large transfer of energy from the exhaust gas flow to thecoolant can therefore be transferred in an advantageous manner.

According to an advantageous refinement of the invention, the exhaustgas guiding tubes are designed in such a manner that a turbulent exhaustgas flow is formed in their interior. The measure of providing aturbulent exhaust gas flow increases the residence time of the exhaustgas in the exhaust gas guiding tube and therefore optimizes the heatexchange within the exhaust gas heat exchanger.

An exhaust gas heat exchanger designed according to the invention, whichmay also involve a development of the abovementioned refinements, makesprovision for the exhaust gas heat exchanger to have at least one tubebody which is formed from a bundle of exhaust gas guiding tubes whichare aligned parallel to one another. In this case, the ends of theexhaust gas guiding tubes are in each case fixed on a common tube plate.They rest here in each case in an opening in the respective tube plateand close this opening in a fluidtight manner. All of the openings inthe tube plate are assigned in each case to one exhaust gas guidingtube. The construction of a tube body in this way can be produced in asimple manner. In addition, the tube body is fixed on account of thismanner of construction and is less susceptible to vibrations.

According to an advantageous refinement, provision is made, in the caseof at least one of the tube plates, for a bell-shaped exhaust gascollector to be arranged on that side of the tube body which faces awayfrom the bundle of tubes, said collector having a connection point andbeing connected in a fluidtight manner to the tube plate. Thebell-shaped exhaust gas collector constitutes a simple means ofproviding a transition between a connection point and the tube body,with a distribution of the exhaust gas flow to the individual exhaustgas guiding tubes of the tube body or a collection of the partial flowsafter they have flowed through the exhaust gas guiding tubes being madepossible.

According to a further advantageous refinement, the exhaust gas heatexchanger has two tube bodies, with two tube plates of the tube bodiesfacing each other—in particular directly butting against each other, theexhaust gas guiding tubes preferably being aligned flush with oneanother—and with said tube plates being connected to each other towardthe outside in a fluidtight manner. The division of the exhaust-gasconducting means into two tube bodies makes it possible to displace thevibration behavior of the exhaust gas guiding tubes in the exhaust gasheat exchanger in the direction of higher natural vibration frequencies.This advantageously results in the service life of the exhaust gas heatexchanger being increased. The natural vibration frequencies aredisplaced into a region in which there are fewer excitations tovibration. This resides in the shortening of the free length of theexhaust gas guiding tubes. As a result, both the vibration behavior anda possible development of noise by the exhaust gas heat exchanger arereduced.

According to a preferred refinement, provision is made here for arespective bell-shaped exhaust gas collector to be arranged at both freeends of two tube bodies connected to each other, said collectors in eachcase having a connection point and being connected to the bundle oftubes in a fluidtight manner. In this case, provision is made inparticular for the coolant channel of the exhaust gas heat exchanger toextend over the length between the two bell-shaped exhaust gascollectors and optionally also to partially enclose them. A respectivesealing device is formed in this case between the bell-shaped exhaustgas collectors and a coolant housing which bounds the coolant channeland radially surrounds the tube bodies. According to a further preferredadvantageous refinement of the coolant guiding housing, the latter iscomposed of two half shells which are added together to form a housingwhich is closed to the outside. The half shells have, on the one hand,two connections which are formed on the end-side of the half shells. Oneof the half shells has a further connection point in particular in theregion of a widening of the outside diameter of the half shells, theconnection point serving for the supply and the withdrawal of coolants.

The coolant is in particular cooling water which has been branched offfrom the cooling circuit of the internal combustion engine and issubsequently supplied to said cooling circuit again. In this case, it ispossible in particular for the cooling liquid then to be supplied to afurther heat exchanger, in particular to the heat exchanger forproducing a flow of warm air for a heating system of the vehicle.Instead of water, the coolant may also be another fluid, for example airwhich is flowing through. Although the use of air reduces the thermalcapacity of the coolant in comparison to other coolants, for examplewater, the heated air produced in this case can be used directly atanother location, for example for defrosting vehicle windows or thelike. Of course, the coolant flow in this case may also be used directlyfor heating the interior of the vehicle.

According to a method according to the invention for producing anexhaust gas heat exchanger, first of all the tube bodies are produced.If required, the two tube bodies are connected to each other in afluidtight manner. The bell-shaped exhaust gas collectors aresubsequently fastened to the free ends of the tube bodies and inparticular are connected to the tube plates in a fluidtight manner. Thetube body, optionally further supported in the region of the exhaust gasguiding tubes by intermediate webs, is inserted into a lower half shell.The sealing devices are fastened to the bell-shaped exhaust gascollectors. They are connected to the lower half shell of the coolantguiding housing in a fluidtight manner. The upper half shell is thenplaced onto the lower half shell. A fluidtight connection is produced inthe region of the sealing device and via the abutting edge of the twohalf shells against each other.

A particularly favorable method for producing two tube bodies, theexhaust gas guiding tubes of which are aligned flush with one another,resides in two tube plates which are arranged tightly next to each otherbeing pushed on to a corresponding number of gas guiding tubes having,as seen continuously, the entire length of the pair of tube bodies. Inthis case, a connection may already be produced between the tube platesand the individual exhaust gas guiding tubes. The connection may beproduced, for example, by shrinking on, by corresponding adhesivebonding or else by welding. Before or after the production of theconnection between the individual exhaust gas guiding tubes and the tubebody, a severing of the bundle of the exhaust gas guiding tubes takesplace in a gap between the two tube plates. Before this severing takesplace, the outer tube plates may also be pushed onto the bundle ofexhaust gas guiding tubes. The severing takes place in particular bylaser cutting or a comparable process. According to a preferredrefinement, the severing may take place in each case flush with thesurface of the two tube plates. If a further working step then has to becarried out in order to fasten the exhaust gas guiding tubes to the tubeplates, then this is carried out now. The two tube plates cansubsequently be connected to each other in a fluidtight manner, whichcan take place in particular by them being adhesively bonded or weldedto each other along their abutting surfaces, with it being possible forthe weld seam to follow the outer contour of the tube plates. However,it is also possible, via an encircling crimping or clamping, to producean additional or sole fastening of the tube plates to each other.Following this, a respective tube plate may also be fastened to the freeends of the exhaust gas guiding tubes, if this has not yet happened. Thebell-shaped gas collectors are welded onto the tube plates.

A sealing device designed according to the invention for an exhaust gasheat exchanger or another fluidtight leadthrough between two components,namely an exhaust-gas-conducting hot component and a cold componentwhich outwardly bounds a cooling device, serves to lead the hotcomponent through to the outside through the cold component. In thiscase, the hot component is held in the cold component in a manner suchthat it can be displaced longitudinally with axial guidance. Accordingto the invention, the sealing device has two sealing elements which areindependent of each other. Each of the sealing elements produces afluidtight connection between hot component and cold component.

The provision of a double seal, with two sealing devices completelyindependent of each other, increases the service life and the securityof the sealing device in particular in the case of applications exposedto high thermal loads. In this case, care has to be taken to ensure thatthe sealing device, on the one hand, is indeed arranged in the region ofthe component cooled by the cooling liquid, but, on the other hand, canbe entirely exposed to the high temperatures of the hot component.

According to a preferred refinement of the invention, an intermediatechamber which is closed in a fluidtight manner is formed between the twosealing elements of the sealing device. This intermediate chamberpermits the seals to be separated spatially from each other and at thesame time to form an insulating gap between the two seals. Furthermore,the penetration of one of the two media, i.e. of the exhaust gas flow orof the coolant flow, into the intermediate chamber between the twosealing devices can constitute an indication of a corresponding leakageand can be monitored.

According to a preferred refinement of the invention, at least one borepassing through the cold component and leading to the outside is fittedinto the sealing device in the region between the two sealing elements,the at least one bore having a thread in particular over a partiallength. The provision of corresponding bores which may lead inparticular into the intermediate chamber results in it being possible ina simple manner firstly to check the seal tightness of the sealingdevices during production and secondly also to monitor the occurrence ofleakages of the sealing devices. It is possible in particular toestablish whether the first, inner sealing element of the sealing devicestill forms a sealing closure in relation to the seal situated betweenthe hot component and the cold component. Medium can only penetrate fromthis chamber into the intermediate space between the two sealingelements if the inner seal is untight. This would be made noticeable bythe escape of medium, for example a coolant, flowing through thechamber. This would escape in this case at the at least one bore andwould be detectable there. According to a preferred refinement of theinvention, the sealing device has more than one, in particular two orthree such bores. The bores are suitable in particular for theconnection of a burst-testing device for carrying out a tightness check.A method for carrying out a tightness check before a device having asealing device designed in such a manner is used involves the boresbeing closed, in particular involves line endings being screwed into thebores. A pressurized fluid, for example compressed air, is then suppliedvia at least one of the lines. It is checked via a pressure measuringdevice connected to another bore whether the pressure provided can bemaintained constantly in the intermediate space over a predeterminedtime without pressurized fluid being fed in. After appropriate venting,the burst-testing device can then be removed again. If a leakage occurs,then it can be checked, for example by measuring the pressure at theconnection points of the chamber of an exhaust gas heat exchangerdesigned according to the invention, whether the leakage takes placeinto the chamber. This can be established with, for example, a rise inpressure in the chamber. It is therefore not only possible by means ofthe burst-testing process to establish leakages, but also to classifywhich of the two sealing elements is not fully sealing. The tightness ofthe coolant channels may also be checked by such a burst-testingprocess.

It is in keeping with advantageous refinements if at least one of thetwo sealing elements is a radial seal or if at least one of the twosealing elements is an axial seal. The terms radial seal and axial sealhave been selected in such a manner that a radial seal refers to a sealin which the sealing means extends in the radial direction from onesealing surface to the other sealing surface and seals off anintermediate space. An axial seal extends along an axis and seals off agap which is correspondingly aligned. Therefore, in the case of theradial seal, the throughflow direction of a leakage is aligned axiallywhile this runs radially in the case of an axial seal. As radial seal,use can be made in particular of sealing rings of polymer compounds orwith metal while the axial seal can be designed, for example, as abellow seal, in which a bellows, which is folded in the manner of aconcertina and forms a ring, is connected in a fluidtight manner by itsaxial ends in each case to one component of the components which are tobe sealed off in relation to one another.

A tightness test during the use of the sealing device, for exampleduring the operation of a vehicle, and the use of the sealing device inan exhaust gas heat exchanger takes place, for example, in such a mannerthat the escape of coolant as a bore is checked. Instead of a visualcheck, it is also possible, of course, for a sensory test to be carriedout via a moisture sensor arranged there. For this purpose, it is inkeeping with an advantageous refinement of the invention if the at leastone bore is arranged in the region of the installation position at thelowest point of the sealing device.

It is in keeping with a particularly advantageous refinement if thesealing device has an axial seal and a radial seal, with the two sealsbeing spaced apart from each other, so that an intermediate chamber isformed in particular between the two sealing elements. In this case, inparticular the axial seal is the inner seal sealing off the two partsfrom each other during operation, while the radial seal is in particularthe back up seal for the situation in which the axial seal has leakages.

The use of an axial seal in conjunction with a leadthrough in the caseof a sealing device according to the invention has the advantage inparticular that said axial seal is suitable for being able more easilyto compensate for length plays between components movable with respectto one another, which length plays occur, for example, due to changes intemperature, than is the case in the case of a radial seal which has tobe able to slide here along a sealing surface, so that an axial strokeof the two parts which are sealed off in relation to each other can becompensated for. The use of two different sealing elements also has theadvantage that, in the case of loads which occur and over the loadcycles, different strengths complement each other.

The invention is furthermore explained in more detail below withreference to the exemplary embodiment which is illustrated in thedrawing, in which:

FIG. 1 shows the sectional illustration through an exhaust gas heatexchanger according to the invention,

FIG. 2 shows the section with reference to the section line B-B throughFIG. 1 in the region of the central connection;

FIG. 3 shows the section according to the section line A-A through FIG.1; and

FIG. 4 shows the enlarged illustration of a detail of a sealing deviceaccording to the invention.

FIG. 1 shows, in a sectional illustration, an exhaust gas heat exchanger10 designed according to the invention. In the illustration of FIG. 1,the exhaust gas heat exchanger 10 has been divided into two parts 10 aand 10 b, with the division having taken place along the dividing line10 c (illustrated by chain-dotted lines), so that the illustration ofthe heat exchanger, which is of linear design per se, can be undertakenon an enlarged scale without having to reproduce the proportions of thetwo tube bodies 23 in a changed manner. FIGS. 2 and 3 show sectionalillustrations along the section lines A-A and B-B illustrated in FIG. 1.FIG. 4 shows an enlarged illustration of a detail in the region of asealing device, as also illustrated in FIG. 1.

An exhaust gas heat exchanger 10 according to the invention, asillustrated in FIGS. 1 to 4, is described below with reference to amethod for producing an exhaust gas heat exchanger of this type.

First of all, a number of exhaust gas guiding tubes 21, which may inparticular be rectangular semi-finished products which can be cut tolength, are aligned in a suitable number parallel to one another inaccordance with the requirements of the invention. For this purpose,first of all the semi-finished products are cut to a length whichcorresponds approximately to the overall length of the section formed bythe exhaust gas guiding tubes 21 in the exhaust gas heat exchanger. Thetwo tube plates 24, which are connected to each other at a later time,are then pushed, slightly spaced apart from each other, onto the exhaustgas guiding tubes 21. In the process, these tube plates 24 arepositioned on the exhaust gas guiding tubes 21 in such a manner that thetwo outwardly emerging, free lengths of the semi-finished product are inaccordance with the desired ratio between the lengths of the exhaust gasguiding tubes of the two tube bodies 23. The end-side tube plates 24 arealso pushed onto the exhaust gas guiding tubes 21. A respective exhaustgas guiding tube rests here in each opening in the tube plate that isintended for receiving an exhaust gas guiding tube. Like the two tubeplates 24 pushed on first of all, the end-side tube plates can beconnected to the exhaust gas guiding tubes 21 by being shrunk on. Ifdesired or required, the end-side tube plates 24 may alternatively oradditionally be welded to the exhaust gas guiding tubes 21 in order toproduce a fluidtight connection to the same. A severing of thesemi-finished products protruding through the central tube plates thentakes place, with it being possible for this severing to be carried out,for example, by laser cutting. The severing takes place in each caseflush with the surface of the mutually facing sides of the two tubeplates 24. Subsequently, if this is still required, the exhaust gasguiding tubes 21 are connected in a fluidtight manner to the twootherwise mutually facing tube plates 24, for example by a correspondingwelding process, with it also being possible for laser welding processesto be used here. In this manner, two tube bodies 23 which form a pairare formed in a simple manner from the semi-finished products.

The mutually facing, inner tube plates 24 are connected to one anotherin a fluidtight manner in the next working step, which takes place, forexample, by them being welded together along their side edge, with carebeing taken to ensure the formation of a fluidtight, continuous weldingseam. The welding can take place by laser welding or by roll spotwelding or another welding process. In order to form a defined outercontour of the two tube plates, the latter can also be engaged around byan encircling ring clip 53 which is in particular also tightly weldedonto it.

Respective bell-shaped exhaust gas collectors 25 are then welded in afluidtight manner on the outer tube plates 24, on the side facing awayfrom the exhaust gas guiding tubes 21. In this case too, an annular clip54 can form the outer contour in the region of the attachment pointbetween tube plate 24 and bell-shaped exhaust gas collector. Also, inparticular welding processes with a weld seam running continuously aresuitable as a fastening possibility for the fastening of the bell-shapedexhaust gas connectors.

The first of two tube bodies 23, which are closed on both sides bybell-shaped exhaust gas collectors 25, is then inserted into a lowerhalf shell 44. The lower half shell forms part of the coolant guidinghousing 13 and in the region of its profile delimits the chamber in theinterior of the coolant guiding housing outward. In the process, theconstructional unit is held in the region of the two tube plates 24which are welded to each other, i.e. not on the edge side in the regionof extent of the constructional unit. The lower half shell 44 has aradial widening 52 which extends axially on both sides of the two tubeplates 24. In the region of the radial widening 52, a bearing ring 47 isformed, said bearing ring radially surrounding the ring clip 53 and thisforming a fixed mount for the constructional unit. By means of thebearing of the ring clip 53 against the bearing ring 47, the path offluid is prevented in the axial direction by the chamber 14. An annularchamber 48 is formed on both sides of the bearing ring 47, only in theregion of the radial widening 52, said annular chamber opening at onepoint into a connection point 19, here a coolant withdrawal connection19, which is fluidically connected to both sides which are separatedfrom each other by the tube plates 24. In the exemplary embodimentillustrated, the coolant withdrawal connection 19, however, is arrangedin the upper half shell 43 which is placed on later.

After the constructional unit is inserted into the lower half shell 44,the sealing unit 30 is fitted. The sealing unit 30 contains a basic bodywhich is of essentially rotationally symmetrical design. The basic bodyends in an insert flange 60 which can be placed against a correspondingbearing flange 49 of the upper half shell 43 and the lower half shell44. The flanges 49 and 60 are connected to each other in a fluidtightmanner by welding, for example. Offset radially inward, the basic body34 has a bearing sleeve 61 which, as seen axially, projects into theregion of the bell-shaped exhaust gas collector 25. In this region, thebearing sleeve 61 is of cylindrical design, i.e. is of rotationallysymmetrical design with a constant diameter, as seen in the axialdirection. Before the basic body 34 is placed on, an intermediate ring62 is welded onto the bell-shaped exhaust gas collector 25 and surroundsthe outer connectors of the bell-shaped exhaust gas collector radiallyin a fluidtight manner. The bellows seal 33 is pushed onto thisintermediate ring 62 axially, said bellows seal having as fasteningpoints a respective annular body 63 at its axial ends, between which thebellows of the bellows seal 33 extends and onto which said bellows isfastened in each case in a fluidtight manner. One of the two annularbodies 63 is welded in a fluidtight manner to the intermediate ring 62.The other annular body 63 has an outer ring 64 which can accommodatedrill holes which are preferably designed as a blind hole. In this case,the outside diameter of the outer ring 64 corresponds approximately tothe outside diameter of the intermediate ring 62, but is spaced apartaxially from the latter, this axial spacing forming the intermediatechamber 35. A pair of O-rings is arranged radially on the outside of theintermediate ring 62, as a radial seal 32 on the circumferentialsurface. After intermediate ring 62 and axial seal 33 are fastened tothe bell-shaped exhaust gas collector 25, the basic body 34 is pushed inthe axial direction over the bell-shaped exhaust gas collectors 25 insuch a manner that the radial seal 32 of the intermediate ring 62 passesinto sealing contact with the inner cylindrical surface of the bearingsleeve 61.

Situated further on the outside in the axial direction, the outer ring64 of the axial seal 33 is radially surrounded by the bearing sleeve 61,with it being possible for the outer contours to have a conical designmatched to one another, said conical designs tapering axially outward.The bearing sleeve 61 merges into a sleeve surface 65 which has borespositioned corresponding to the bores in the outer ring 64 and via whicha screw fastening with fastening screws 66 can take place, with afluidtight end being achieved via the in particular form-fitting bearingof the outer ring 64 against the sleeve surface 65 in the cylindrical toconical inner surface of the bearing sleeve 61. In this case, the sleevesurface 65 opens up an opening into which a connecting flange 67 isinserted, said connecting flange being designed as a sleeve and the freeinside diameter of which, through which the flow can flow, correspondsto the diameter of the bell-shaped exhaust gas collector in its inflowregion. That inner region of the bell-shaped exhaust gas collectorthrough which the exhaust gas flows is separated from the chamber 14 inthe interior of the coolant guiding housing 13 via the axial seal 33 andthe radial seal 32. Owing to the fact that the axial seal 33 has axialmovability in the axial direction via the convolution of its bellows andthe intermediate ring 62 can slide in the axial direction in the bearinghousing 61, an axial compensation of the thermal expansion of the twotube bodies 23 is possible. The leadthrough of the exhaust gas supplyline 16 or of the exhaust gas withdrawal line 17 through the exhaust gasheat exchanger is therefore a loose bearing, as seen in the axialdirection. Only in the region of the bearing ring 47 is a positionallyfixed mounting of the tube bodies 23 and therefore of all of the hotcomponents out of the cold components provided.

In this case, the hot component 40 is the bell-shaped exhaust gascollector 25 and the connecting flange 67 forming a connection point 15.The cold component 41 is formed by the upper and lower half shells 43,44 and by the basic body 34 of the sealing device 30.

Bores 36 are guided through the basic body 34 into the intermediatechamber 35 which is bounded in the axial direction by the intermediatering 62 and the annular body 63 or the ring 64 and radially by the axialseal and the bearing sleeve 61. If the radial seal 32, which is formedhere from two O-rings independent of each other, has a leakage, thenliquid flows out of the chamber 14 into the intermediate chamber 35 dueto this leakage. In the interior of the intermediate chamber 35, thisliquid then flows to the lowest point by a bore 36 being provided ineach case. The escape of liquid from this bore enables the presence of aleakage to be established, but an overflow of the escaping coolingliquid, for example cooling water, into the part conducting exhaust gasis not possible owing to the presence of the axial seal 33.

The connection points for the coolant supply connection 18 are alsoformed in the lower half shell 44. These connection points are formed ina region of the lower half shell 44 by the latter also surrounding partof the bell-shaped exhaust gas collector 25, so that the chamber 14formed in the interior of the coolant guiding housing 13 which forms issituated and is therefore in contact with the influence of the coolant.

In the next working step, the upper half shell 43 is placed onto thelower half shell 44 and the half shells are welded and screwed to eachother in a fluidtight manner. In this case, the upper half shell alsohas a bearing flange 49. The basic body 34 is connected to the sealingdevice 30 in a fluidtight manner, i.e. in particular is welded thereto.It is also possible to provide screw bores in the direction of extent ofthe tube bodies 23, by means of which the two half shells can beconnected to each other, with it also being possible to provide a sealin order to obtain a fluidtight casing. It is possible to hold thebearing flange 60 in a fluidtight manner on the bearing flange 49 by ascrew connection, with it also being possible for sealing means, such asO-rings, to be used here as sealing means.

The exhaust gas heat exchanger 10 illustrated in FIG. 1 has an exhaustgas flow flowing through it, illustrated diagrammatically by the arrows11. The exhaust gas supply line 16 supplies the exhaust gas flow whichthen flows into a bell-shaped exhaust gas collector 25 which acts as adiffuser and distributes the exhaust gas flow to the individual exhaustgas guiding tubes 21 of the tube bodies 23. After flowing through thetube bodies, the exhaust gas flow 11 is collected in the secondbell-shaped exhaust gas collector 25 and leaves the exhaust gas heatexchanger 10 in the exhaust withdrawal line 17.

The exhaust gas heat exchanger has a cooling liquid, such as water,flowing through it as coolant. The coolant flow is illustrated by thearrows 12. Two coolant flows form in the exhaust gas heat exchanger. Thecoolant is supplied through the two coolant supply connections 18 formedin the region of the bell-shaped exhaust gas collectors 25. It flows ineach case from the coolant supply connection 18 to the coolantwithdrawal connection 19 formed in the region in which the two bundlesof tubes 25 meet. From the coolant supply connection 18, which is formedon the supply side of the exhaust gas flow, i.e. in the region of theexhaust gas supply line 16, the coolant flows parallel to the exhaustgas flow flowing through the exhaust gas guiding tubes 21, to thecoolant withdrawal connection 19. This is the shorter region in theembodiment illustrated, the length of which is approximately half thesize of the longer of the two regions. In this case, the coolant flow isguided in the chamber 14, which is bounded by the coolant guidinghousing 13, and flows around the exhaust gas guiding tubes 21 on allsides, so that a good flow of heat from the exhaust gas flow to thecoolant flow is ensured. The coolant flow, which flows through theexhaust gas heat exchanger in the opposite direction to the direction offlow through the tube bodies 23, flows from the coolant supplyconnection 18—which is formed in the region of the bell-shaped exhaustgas collector 25 opening into the exhaust gas withdrawal line—to theannular chamber 48 and likewise passes into the coolant withdrawalconnection 19 where it leaves the exhaust gas heat exchanger 10.

During the flow through the exhaust gas heat exchanger, the exhaust gasflowing through is cooled while the coolant is heated. The exchange ofthermal energy is facilitated by a surface area which is as large aspossible and in which a contact between a separating surface, which hasas thin a wall as possible, the exhaust gas flow is thermallyconductively in contact with the coolant flow.

In this case, a process according to the invention, for cooling anexhaust gas flow, is complied with if the direction in which the coolantflows past the exhaust gas flow is divided into two regions, with theexhaust gas flow being oriented initially parallel to the exhaust gasflow in one region, for example the hotter of the two regions, while, ina second region, the coolant flow is directly counter to the throughflowdirection of the exhaust gas flow. The two regions can be selected in anadvantageous manner such that the thermal linear expansion of theexhaust gas guiding tubes corresponds to one another in these tworegions. This makes it necessary for the first region to be of shorterdesign because a more rapid cooling is formed in this region than in theother region, with the effect ultimately being achieved by the differentoverall length that the difference in temperature of the two regionsessentially corresponds to each other.

The invention claimed is:
 1. A sealing device for exhaust gas heatexchangers, the sealing device being formed between anexhaust-gas-conducting hot component and a cold component whichoutwardly bounds a cooling device, wherein part of the hot component isguided outward from the cooling device, and with the hot component beingheld in the cold component in a manner such that the hot component canbe displaced longitudinally with axial guidance, the sealing devicecomprising: two sealing elements which are independent of each other,each of the two sealing elements producing a fluidtight connectionbetween the hot component and the cold component, the two sealingelements including a first sealing element and a second sealing element;an intermediate ring, the intermediate ring having a recess toaccommodate the first sealing element therein; and two annular bodies,the second sealing element being positioned, in a fluid-tight manner,between the two annular bodies, wherein a first one of the two annularbodies is welded, in a fluid-tight manner, to the intermediate ring,such that the first one of the two annular bodies directly contacts theintermediate ring.
 2. A sealing device for exhaust gas heat exchangers,the sealing device being formed between an exhaust-gas-conducting hotcomponent and a cold component which outwardly bounds a cooling device,wherein part of the hot component is guided outward from the coolingdevice, and with the hot component being held in the cold component in amanner such that the hot component can be displaced longitudinally withaxial guidance, the sealing device comprising two sealing elements whichare independent of each other, wherein each of the two sealing elementsproduces a fluidtight connection between the hot component and the coldcomponent, wherein an intermediate chamber, which is bounded by the hotcomponent and the cold component and is closed in a fluidtight manner bythe two sealing elements, is formed between the two sealing elements,and wherein the cold component, in the intermediate chamber between thetwo sealing elements, has at least one bore passing through the coldcomponent and leading outward, which bore has a thread preferably atleast over a partial length.
 3. The sealing device as claimed in claim2, wherein the sealing device has two or three bores which are suitablefor the connection of a burst-testing for carrying out a tightnesscheck.
 4. The sealing device as claimed in claim 1, wherein the firstsealing element is a radial seal.
 5. The sealing device as claimed inclaim 1, wherein the second sealing element is an axial seal.
 6. Thesealing device as claimed in claim 1, wherein the first sealing elementis an axial seal and the second sealing element is a radial seal, withthe axial seal and the radial seal being spaced apart from each other.7. The sealing device as claimed in claim 5, wherein the axial seal is abellows seal.
 8. The sealing device as claimed in claim 1, furthercomprising an outer ring, the outer ring being spaced apart from theintermediate ring in an axial direction, and the outer ring directlycontacting a second one of the two annular bodies.
 9. A sealing devicefor exhaust gas heat exchangers, the sealing device being formed betweenan exhaust-gas-conducting hot component and a cold component whichoutwardly bounds a cooling device, wherein part of the hot component isguided outward from the cooling device, and with the hot component beingheld in the cold component in a manner such that the hot component canbe displaced longitudinally with axial guidance, the sealing devicecomprising two sealing elements which are independent of each other,wherein each of the two sealing elements produces a fluidtightconnection between the hot component and the cold component, and whereinan intermediate chamber, provided between the two sealing elements, hasat least one bore passing through the cold component and leadingoutward.